线程之间的通信问题：生产者和消费者问题！

1.生产者和消费者问题 Synchronized 版

线程交替执行 A   B 操作同一个变量

/**
 * 线程之间的通信问题：生产者和消费者问题！  等待唤醒，通知唤醒
 * 线程交替执行  A   B 操作同一个变量   num = 0
 * A num+1
 * B num-1
 */
public class A {
    public static void main(String[] args) {
        Data data = new Data();

        new Thread(()->{
            for (int i = 0; i < 10; i++) {
                try {
                    data.increment();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        },"A").start();

        new Thread(()->{
            for (int i = 0; i < 10; i++) {
                try {
                    data.decrement();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        },"B").start();

        new Thread(()->{
            for (int i = 0; i < 10; i++) {
                try {
                    data.increment();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        },"C").start();


        new Thread(()->{
            for (int i = 0; i < 10; i++) {
                try {
                    data.decrement();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        },"D").start();
    }
}

// 判断等待，业务，通知
class Data{ // 数字 资源类

    private int number = 0;

    //+1
    public synchronized void increment() throws InterruptedException {
        if (number!=0){  //0
            // 等待
            this.wait();
        }
        number++;
        System.out.println(Thread.currentThread().getName()+"=>"+number);
        // 通知其他线程，我+1完毕了
        this.notifyAll();
    }

    //-1
    public synchronized void decrement() throws InterruptedException {
        if  (number==0){ // 1
            // 等待
            this.wait();
        }
        number--;
        System.out.println(Thread.currentThread().getName()+"=>"+number);
        // 通知其他线程，我-1完毕了
        this.notifyAll();
    }

}

上述代码存在一个问题：

如果A B C D 4 个线程，就会出现问题，就是所谓的虚拟唤醒！

怎么解决呢？

将其中的if else判断改成while判断即可。

/**
 * 线程之间的通信问题：生产者和消费者问题！  等待唤醒，通知唤醒
 * 线程交替执行  A   B 操作同一个变量   num = 0
 * A num+1
 * B num-1
 */
public class A {
    public static void main(String[] args) {
        Data data = new Data();

        new Thread(()->{
            for (int i = 0; i < 10; i++) {
                try {
                    data.increment();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        },"A").start();

        new Thread(()->{
            for (int i = 0; i < 10; i++) {
                try {
                    data.decrement();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        },"B").start();

        new Thread(()->{
            for (int i = 0; i < 10; i++) {
                try {
                    data.increment();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        },"C").start();


        new Thread(()->{
            for (int i = 0; i < 10; i++) {
                try {
                    data.decrement();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        },"D").start();
    }
}

// 判断等待，业务，通知
class Data{ // 数字 资源类

    private int number = 0;

    //+1
    public synchronized void increment() throws InterruptedException {
        while (number!=0){  //0
            // 等待
            this.wait();
        }
        number++;
        System.out.println(Thread.currentThread().getName()+"=>"+number);
        // 通知其他线程，我+1完毕了
        this.notifyAll();
    }

    //-1
    public synchronized void decrement() throws InterruptedException {
        while (number==0){ // 1
            // 等待
            this.wait();
        }
        number--;
        System.out.println(Thread.currentThread().getName()+"=>"+number);
        // 通知其他线程，我-1完毕了
        this.notifyAll();
    }

}

2.JUC版的生产者和消费者问题

先看一个对比

通过 Lock 找到 Condition

import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;

public class B  {
    public static void main(String[] args) {
        Data2 data = new Data2();

        new Thread(()->{
            for (int i = 0; i < 10; i++) {
                try {
                    data.increment();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        },"A").start();

        new Thread(()->{
            for (int i = 0; i < 10; i++) {
                try {
                    data.decrement();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        },"B").start();

        new Thread(()->{
            for (int i = 0; i < 10; i++) {
                try {
                    data.increment();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        },"C").start();

        new Thread(()->{
            for (int i = 0; i < 10; i++) {
                try {
                    data.decrement();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        },"D").start();

    }
}

// 判断等待，业务，通知
class Data2{ // 数字 资源类

    private int number = 0;

    Lock lock = new ReentrantLock();
    Condition condition = lock.newCondition();

    //condition.await(); // 等待
    //condition.signalAll(); // 唤醒全部
    //+1
    public void increment() throws InterruptedException {
        lock.lock();
        try {
            // 业务代码
            while (number!=0){  //0
                // 等待
                condition.await();
            }
            number++;
            System.out.println(Thread.currentThread().getName()+"=>"+number);
            // 通知其他线程，我+1完毕了
            condition.signalAll();
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            lock.unlock();
        }

    }

    //-1
    public synchronized void decrement() throws InterruptedException {
        lock.lock();
        try {
            while (number==0){ // 1
                // 等待
                condition.await();
            }
            number--;
            System.out.println(Thread.currentThread().getName()+"=>"+number);
            // 通知其他线程，我-1完毕了
            condition.signalAll();
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            lock.unlock();
        }
    }

}

 3.Condition 精准的通知和唤醒线程

任何一个新的技术，绝对不是仅仅只是覆盖了原来的技术，优势和补充！

可以看到上面的执行结果都是随机的，那么我要让他ABCD顺序执行，该怎么操作呢？

import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;

/**
 * @author 狂神说Java 24736743@qq.com
 * A 执行完调用B，B执行完调用C，C执行完调用A
 */
public class C {

    public static void main(String[] args) {
        Data3 data = new Data3();

        new Thread(()->{
            for (int i = 0; i <10 ; i++) {
                data.printA();
            }
        },"A").start();

        new Thread(()->{
            for (int i = 0; i <10 ; i++) {
                data.printB();
            }
        },"B").start();

        new Thread(()->{
            for (int i = 0; i <10 ; i++) {
                data.printC();
            }
        },"C").start();
    }

}

class Data3{ // 资源类 Lock

    private Lock lock = new ReentrantLock();
    private Condition condition1 = lock.newCondition();
    private Condition condition2 = lock.newCondition();
    private Condition condition3 = lock.newCondition();
    private int number = 1; // 1A  2B  3C

    public void printA(){
        lock.lock();
        try {
            // 业务，判断-> 执行-> 通知
            while (number!=1){
                // 等待
                condition1.await();
            }
            System.out.println(Thread.currentThread().getName()+"=>AAAAAAA");
            // 唤醒，唤醒指定的人，B
            number = 2;
            condition2.signal();
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            lock.unlock();
        }
    }

    public void printB(){
        lock.lock();
        try {
            // 业务，判断-> 执行-> 通知
            while (number!=2){
                condition2.await();
            }
            System.out.println(Thread.currentThread().getName()+"=>BBBBBBBBB");
            // 唤醒，唤醒指定的人，c
            number = 3;
            condition3.signal();
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            lock.unlock();
        }
    }
    public void printC(){
        lock.lock();
        try {
            // 业务，判断-> 执行-> 通知
            // 业务，判断-> 执行-> 通知
            while (number!=3){
                condition3.await();
            }
            System.out.println(Thread.currentThread().getName()+"=>BBBBBBBBB");
            // 唤醒，唤醒指定的人，c
            number = 1;
            condition1.signal();
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            lock.unlock();
        }
    }

}